Wireless local area networks (WLANs) are currently most commonly implemented according to the Institute of Electrical and Electronic Engineers (IEEE) 802.11-1999 standard, often referred to as “wireless fidelity” or “WiFi”. A number of working groups are currently developing modifications and extensions to the standard for various purposes. In particular, the IEEE 802.11 Working Group's Medium Access Control (MAC) Enhancements task group (Task Group E) is working to incorporate quality of service (QoS) into wireless local area networks for high quality delivery of video, voice and multimedia (see IEEE 802.11e QoS draft D2.0a, November 2001), while the IEEE 802.11 Task Group H (TGh) is developing a standard for incorporation of dynamic frequency selection and transmit power control into IEEE 802.11a implementations (see “DFS and TPC Joint Proposal for 802.11h”, IEEE 802.11-01/169r2).
Energy consumption has become an important issue in IEEE 802.11 wireless networks since mobile terminal battery life is limited. Energy conservation may be achieved in several ways, one of which is utilizing transmit power control (TPC), a mechanism reducing energy consumption and co-channel interference by adapting the radio transmit power to the minimum level required to achieve a desired link reliability (i.e., to guarantee correct reception of a frame). As noted above, the IEEE 802.11h Working Group is developing TPC for 802.11a, and similar algorithms are expected for the 802.11b physical (PHY) layer.
Use of the TPC mechanism in an IEEE 802.11e network operating under the hybrid coordination function (HCF) would be desirable, particularly since direct communication between quality-of-service (QoS) stations (QSTAs) without external control is permitted—that is, one QSTA can transmit to another without first transmitting to the access point/hybrid coordinator (AP/HC). Presumptively individual stations (STAs) would control transmit power during each contention-free period (CFP) and contention-free burst under the HCF operation.
Under one proposed implementation for the HCF, however, the hybrid coordinator (HC) must “hear” all frames in the CFB/CFP. Otherwise the HC, sensing the medium as idle for a distributed coordination function (DCF) inter-frame spacing (DIFS) period, will reclaim the channel and attempt to transmit. In addition, the HC needs to receive certain frames (e.g., to update queue sizes) for correct operation of the QoS-supporting basic service set (QBSS) network. Therefore every QSTA in a QBSS network should transmit every frame at a high enough power so that the HC can hear the frame, which considerably reduces the benefits of TPC. In QSTA-to-QSTA communications, the transmit power may not be as low as possible.
There is, therefore, a need in the art for improving the adaptation of transmit power control to hybrid coordination function operation under quality-of-service supporting basic service set networks.